<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <style>
    .box {
      width: 300px;
      border: 1px solid #000;
    }
    .child {
      padding-top: 100%;
    }
  </style>
  <body>
    <div class="box">
      <div class="child"></div>
    </div>
  </body>
</html>
